The U.S. <strong>Climate</strong> <strong>Change</strong> <strong>Science</strong> <strong>Program</strong>those explorations suggest that it should not bediscounted. Shin et al. (2006) prescribed somesubjectively reconstructed vegetation changes(e.g., Diffenbaugh and Sloan, 2002) in theirAGCM simulations and noted a reduction inspring and early summer precipitation (thatcould carry over into reduced soil moistureduring the summer), but also noted a variableresponse in precipitation during the summer tothe different vegetation specifications. Wohlfahrtet al. (2004) asynchronously coupled anequilibrium global vegetation model, Biome 4(Kaplan et al., 2003), to an AOGCM and observeda larger expansion of grassland in thosesimulations than in ones without the vegetationchange simulated by the EGVM. Finally, Gallimoreet al. (2005) examined simulations usingthe fully coupled AOVGCM (FOAM-LPJ),and while the overall precipitation change forsummer was weakly negative, the impact of thesimulated vegetation change (toward reducedtree cover at 6 ka), produced a small positiveprecipitation change.An analysis currently in progress with RegCM3suggests that the inclusion of the observedmiddle Holocene vegetation in the boundaryconditions for the 6 ka simulation describedabove (Diffenbaugh et al., 2006) further amplifiesthe negative summer precipitation anomalyin the core region of the Holocene drought, andalso alters the nature of the seasonal cycle of thedependence of soil moisture on precipitation.The magnitude of the drought in these simulationsis relatively close to that inferred from thepaleoenvironmental data.The North American midcontinental droughtduring the middle Holocene thus provides anillustration of a significant hydrologic anomalywith relatively abrupt onset and ending thatoccurred in response to gradual changes in themain driver of Holocene climate change (insolation),reinforced by regional- and continentalscalechanges in atmospheric circulation relateddirectly to deglaciation. As was the case forthe African humid period, feedback from thevegetation change that accompanied the climatechanges could be important in reinforcing oramplifying the climate change, and work isunderway to evaluate that hypothesis.There are other examples of abrupt hydrologicalresponses to gradual or large-scale climaticchanges during the Holocene. For example,the development of wetlands in the NorthernHemisphere began relatively early in the courseof deglaciation but accelerated during the intervalhigh summer insolation between 12 ka and8 ka (Gajewski et al., 2001; MacDonald et al.,2006). The frequency and magnitude of floodsacross a range of different watershed sizes alsotracks climate variations during the Holocene(Fig. 3.13J; Knox 1993, 2000; Ely, 1997), albeitin a complicated fashion, owing to dependenceof flooding on long-term climate and land-coverconditions as well as on short-term meteorologicalevents (see Sec. 6).4.4 Century-Scale HydrologicVariationsHydrologic variations, many abrupt, occur ontime scales intermediate between the variationsover millennia that are ultimately related toorbitally governed insolation variations and theinterannual- to decadal-scale variations documentedby annual-resolution proxy records. Asample of time series that describe hydrologicvariations on decadal-to-centennial scales overthe past 2,000 years in North America appearsin Figure 3.15 and reveals a range of differentkinds of variation, including:• generalized trends across several centuries(Fig. 3.15C,F,G);• step-changes in level or variability (independentof sampling resolution) (Fig. 3.15A,B,F);• distinct peaks in wet (Fig. 3.15A) or dryconditions (Fig. 3.13F; Fig. 3.15B,G);• a tendency to remain persistently above orbelow a long-term mean (Fig. 3.15C–F),often referred to as “regime changes”; and• variations in all components of the hydrologiccycle, including precipitation, evaporation,storage, and runoff, and in water quality(e.g., salinity).Hydrological records that extend over the lengthof the Holocene, in particular those from hydrologicallysensitive speleothems, demonstratesimilar patterns of variability throughout (e.g.,Asmerom et al., 2007), including long-termChapter 3104
Abrupt <strong>Climate</strong> <strong>Change</strong>(Anderson et al., 2005)(Benson et al., 2002)(Asmerom et al., 2007)(Meko et al., 2007)(Cook et al., 2004)(Laird et al., 1996)(Booth et al., 2006)Figure 3.15. Representative hydrological time series for the past 2,000 years. A,oxygen-isotope composition of lake-sediment calcite from Jellybean Lake, AK, anindirect measure of the strength of the Aleutian Low, and hence moisture (Andersonet al., 2005). B, oxygen-isotope values from core PLC97-1, Pyramid Lake, NV,which reflect lake-level status (Benson et al., 2002); C, oxygen-isotope values froma speleothem from the Guadalupe Mountains, NM, which reflect North Americanmonsoon-related precipitation (Asmerom et al., 2007); D, dendroclimatologicalreconstructions of Colorado River flow (Meko et al., 2007); E, area averages forthe Western United States of dendroclimatological reconstructions of PDSI (PalmerDrought Severity Index, Cook et al., 2004); F, diatom-inferred salinity estimates forMoon Lake, ND, expressed as deviations from a long-term average (Laird et al., 1996);G, depth-to-water-table values inferred from testate amoeba samples from a peatcore from Minden Bog, MI (Booth et al., 2006). Abbreviations: ‰, per mil; m 3 y –1 ,cubic meters per year; g l –1 , grams per liter; cm, centimeter.105
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